1. Field of the Invention
The present invention relates to an optical fiber having a small bending loss.
2. Description of the Related Art
An electric transmission and optical interconnection are two representative methods for transmitting a signal in an apparatus.
Along with recent acceleration of a clock frequency of a central processing unit (CPU), the electric transmission has a problem that a cross-talk occurs due to a high density interconnection, necessitating an application of a waveform shaping technique to the electric transmission. As a result, it is discovered that if the electric transmission is adopted as the signal transmission in the apparatus, the transmission is limited to a transmission distance of 1 meter and a transmission rate of about 10 Gigabits per second (Gbps).
Meanwhile, the optical interconnection is capable of transmitting a signal in a far broader bandwidth than that of the electric transmission, and constructing a signal transmission system using small-sized, low power consumption optical components. Attention has been, therefore, paid to the optical interconnection as an inter-apparatus signal transmission technique.
Furthermore, as an optical transmission means for the optical interconnection, attention has been paid to a technique using an optical fiber. It is desirable to save a space for storing all optical components employed to make a size of the apparatus compact. As for the optical fiber for the optical interconnection, a flexible wiring capability and a low splicing loss is highly desired.
As an uncooled light source for an access system, Ethernet®, or a fiber channel, attention has been paid to a vertical-cavity surface-emitting laser (VCSEL) that operates at a direct modulation rate of 2.5 Gbps to 10 Gbps. Compared with an edge-emitting laser such as a distributed feedback (DFB) laser, the VCSEL has features of emitting a laser beam vertically from a surface of a substrate, easy formation of a multi-channel array, operation at a low threshold and low power consumption, a high reflectivity of an emitting surface and strong property against a return light (isolator-free laser), and a circular beam shape for an easy coupling with an optical fiber with a high coupling efficiency (lens-free laser).
Since the number of components such as the isolator and the lens can be reduced, the VCSEL is a device that can realize cost reduction of a module. The VCSEL using a GaAs/AlGaAs quantum well layer as an active layer and operating in a wavelength band of 850 nm has been already spread widely as a de facto standard of a laser for a short-haul communication. Examples of the typical optical fiber for this purpose include a silica-based grated-index optical fiber that is one type of a multimode fiber (MMF).
The MMF characteristically has a core diameter about ten times as large as that of a single-mode fiber (SMF) with a large numerical aperture. Accordingly, the MMF does not require high accuracy for splicing the optical components, e.g., splicing optical fibers to each other or splicing the optical fiber to a light source, which ensures an easy splicing.
In recent years, with a view of realizing a higher rate transmission, use of the SMF having a lower loss and a wider bandwidth, as compared with the MMF, has been considered. As a light source, attention is paid to the VCSEL having an oscillation wavelength in a wavelength band of 1.3 μm (1300±50 nm) that is a bandwidth in which the silica-based optical fiber shows a low loss, and the VCSEL is actively researched and developed.
However, if the standard SMF specified in International Telecommunication Union Telecommunication Standard Sector (ITU-T) G.652 is used, a large bending loss occurs when it is stored in an apparatus with a predetermined radius of curvature. Therefore, the standard SMF can hardly be employed in the optical interconnection system.
As the SMF having an improved bending loss, an optical fiber having a trench type refractive index profile and having a lower diffractive index portion than a cladding provided in the cladding of the standard SMF is reported as an optical fiber suitably used in an Fiber-to-the-Home (FTTH) system, as described in Fujikura Ltd., Optics and Electronics Laboratory, Optical Process Research Section, M. IKEDA, S. MATSUO, and K. HIMENO, “Low Bending Loss Optical Fiber with Reduced Splice Loss”, Technical report of the Institute of Electronics Information and Communications Engineer (IEICE), 0CS2003-43, OFT2003-25 (2003-8). However, the SMF is still unsatisfactory for the use in the optical interconnection system.
As explained above, as the optical fiber used in the optical interconnection system, a demand rises for realizing an optical fiber capable of reducing both the bending loss and the splicing loss, ensuring high rate optical transmission, and facilitating construction of the optical interconnection system.